Tough fracture

The glass-fibre nylons have a resistance to creep at least three times as great as unfilled polymers. In the case of impact strength the situation is complex since unfilled nylons tend to break showing tough fracture whereas the filled polymers break with a brittle fracture. On the other hand the glass-filled polymers are less notch sensitive and in some tests and service conditions the glass-filled nylons may prove the more satisfactory. 

Strictly the terms brittle and tough fracture can only be applied to failure under carefiilly specified test conditions. That is to say that the statement that a glassy polymer, such as poly(methyl methacrylate), undergoes brittle fracture at ambient temperatures needs qualifying test conditions must be stated. These are usually that the material has been formed into a dumbbell shaped specimen. 

The phenomena of brittle and tough fracture give rise to fairly characteristic stress-strain curves. Brittle fracture in materials leads to the kind of behaviour illustrated in Figure 7.1 fairly uniform extension is observed with increasing stress, there is minimal yield, and then fracture occurs close to the maximum on this graph. 

Tough fracture, which is alternatively known as ductile fracture, by contrast, gives the type of behaviour illustrated in Figure 7.2. After the maximum in the stress-strain plot has been reached, there is a substantial amount of yielding, before the sample eventually breaks. 

The large region of yield in materials that fail by tough fracture arises as the molecules of the polymer rearrange themselves in response to the applied stress. This is different from the mechanism of yield in metals, where planes of metal atoms slide over one another. In polymers, the molecular movement... 

Figure 7.2 Stress-strain plot for tough fracture...

Bradley, W.L. (1990). The effeet of resin toughness on delamination toughness and post-impact compression strength. In Proc. "Benihana" Intern. Symp. How to improve the toughness of polymers and composites-toughness, fracture and fatigue of polymers and composites, Yamagata. Japan, pp. 221-230. 

A properly designed glass-ceramic is pore-free, mechanically strong (modulus of rupture 250 MPa) and tough (fracture energy 17.1 nr2). 

Novel, toughened one-component epoxy structural adhesives based on epoxy-terminated polyurethane prepolymer incorporating an oxolidone structure were developed to provide improved toughness, fracture resistance and adhesive properties with good chemical and moisture resistance.21 The hybrid resin cures with a standard latent curing agent/accelerator. 

Dynamic properties, fracture toughness, fracture energy, dynamic modulus, dynamic testing, impact tests, drop tower, high strain rate. 

Concrete structure. Retrofitting, Carbon fiber sheet. Adhesive interface, Mixed mode. Fracture toughness. Fracture criterion... 

Short fibre composites, fibre orientation, fracture toughness, fracture mechanisms, fibre pullout, fibre debonding, critical fibre angle. 

Figure 10. TEM image of stained PA-polybutadiene (15 vol%) after undergoing tough fracture in a notched tensile test at high speed (1 m/s). The section shown is next to the fracture surface (18).

Figure 13. Schematic of the structure of the stress-whitened zone perpendicular to the fracture plane, a Tough fracture at high speed, b Tough fracture at low speed.

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